Free space optics are commonly used to couple optical beams between two pre-positioned waveguides. Take for example chip-to-chip coupling, where a first optical chip such as a laser produces a beam that is to be coupled to a second optical chip such as a modulator. In such an environment, the free space optical couplers are discreet optical elements positioned between the chips. Although common, accurate free space coupling in this manner is very difficult to achieve. And, as more and more optical chips are integrated into a single optical module or device, the desire for free space optics systems that allow quick, reliable and affordable coupling between waveguides greatly increases. Unfortunately, current assembly techniques have many shortcomings.
In a typical chip-to-chip assembly, the optical chips need to have a direct path for thermal heat dissipation. Laser diodes in particular dissipate power that must be evacuated to avoid performance degradation. The need for heat dissipating support structures prevents the mounting of optical chips onto compliant structures that allow for chip movement or chip location adjustability. Instead of a compliant structure, these chips are mounted directly to a heat sinking substrate, typically via a pick-and-place assembly in which the locations of the chips have already been pre-programmed. Because the chips are usually assembled first into the device and their position accuracy is dependant on the dimensional accuracy of various components and the accuracy of the placement process, the free space optics coupling beams between chips must be positioned based on the actual locations of the chips.
So, not only are the free space optics positioned to receive the optical beam from the actual location of a first chip, these optics are positioned to focus that optical beam onto the actual location of the second chip. If, during assembly, the free space optical components are shifting from their optimal positions, chip-to-chip coupling efficiency is greatly reduced and device performance will be severely hampered. Such misplacement or misalignment of the free space optics can result from numerous factors, including inaccurate initial optical components placement due, for example, to accuracy limitations of the pick-and-place assembly tooling. Misalignment may also be affected by process shifts such as post-weld shift or epoxy curing shift, e.g., the shifting of the optical component due to laser welding or epoxy curing that is typically used to affix these structures to a support substrate.
Some free space optics techniques using a combination of strong and weak lenses have been developed. Although somewhat well-suited for chip-to-chip coupling, these technologies are sub-optimal in that they require very precise alignments of all optical chips, which add considerably to the fabrication cost. Using strong and weak lenses also results in rather bulky optical devices.
FIG. 1 shows a side view of an existing strong and weak lens coupling apparatus 100. Two waveguides, one of a laser diode 102 and the other of a modulator 104, are coupled together through a strong lens 106 and a weak lens 108. The strong lens 106 and the weak lens 108 form a free space optics system 109. An optical isolator 110 is placed in the optical path between the two lenses 106 and 108 to prevent back-reflected light from entering the laser diode 102.
The ratio of the displacement of the beam waist to the displacement of the strong lens 106 is large, by design. Also, the ratio of the displacement of the beam waist to the displacement of the weak lens 108 is small (i.e., large lens motion→small beam waist motion). In other words, the movement of the weak lens 108 has a lesser effect on the location of the focused optical beam from the laser 102 than does the movement of the strong lens 106. The laser diode 102 and the modulator 104 are mounted on their own submounts 112 and 114, respectively. The strong lens 106 and the isolator 110 may be mounted on micro-flexure 116 and the weak lens 108 on a micro-flexure 118. The two micro flexures 116 and 118 are attached to a weld plate 120.
The assembly process for the device 100 is typically as follows. The submounts 112 and 114 and the chips 102 and 104 are bonded to the substrate at substantially predefined locations. The strong and weak lenses 106, 108, respectively, are aligned simultaneously and the coupling between the two waveguide (chips) is maximized. The alignment step is complicated, because alignment must be optimized for 6 degrees of freedom, all at once. After alignment, the strong lens 106 is attached, and any shift of the strong lens 106 during the attachment process, i.e., post-weld shift, will shift the focus point of the system 109, thereby significantly reducing the coupling efficiency. The weak lens 108 is then realigned so that the focus point is re-centered on the coupled waveguide (a modulator for example) 104, so coupling efficiency is improved. This process may or may not achieve maximum coupling between the devices 102 and 104, however. The misalignment of the strong lens 106 may permanently degrade beam coupling. In any event, after the re-centering, the weak lens 108 is then attached. And, here, a shift of the weak lens 108 would result in an additional shift of the focus point and further coupling loss. In part because the lens 106 and 108 have curved surfaces, for either of the lens the amount of focus point shift as a result of lateral movement of the lens is substantial.